Managing the Irregular Cornea
with Contact Lenses

BY RANDY McLAUGHLIN, O.D., M.S.
AUG. 1997

Don't let an irregular cornea deter you in your quest for best corrected vision. Use a systematic approach and tap all resources.

Corneal irregularities, whether a result of keratoconus, penetrating keratoplasty, trauma, infection or refractive surgery, can test the mettle of any contact lens practitioner. Yet, achieving best corrected vision in these patients can be very rewarding, resulting in long-standing patient loyalty and even referrals for conventional contact lenses.

KERATOCONUS: TIMELY MANAGEMENT IS KEY

Keratoconus is a corneal ectasia that results in irregular astigmatism. Keratometry provides useful fitting information in the early to moderate stages of this condition. Extend the keratometry reading by placing a +1.25D trial lens over the opening of the keratometer and then using the extension conversion chart. Corneal mapping devices are even better for interpreting the corneal topography in the initial (Fig. 1) to later stages of this disease.

Trial fitting with a known lens set is mandatory, and there are several excellent keratoconus lens fitting sets: Soper, McGuire, Contex Aspheric K-18, VFL, Rose K and CLEK (Table 1). Choose the initial base curve slightly steeper than the midpoint of the two keratometric readings. Place this lens on the eye and evaluate the lens-to-cornea fitting relationship with fluorescein. Look for central apical touch (Fig. 2).

The art of fitting the keratoconus patient is to bracket lens vaults, that is, to change the base curve-to-cornea relationship in 0.1mm increments until central lens touch is minimized without allowing excessive edge standoff or significant residual bubble entrapment (Fig. 3). Once you select a base curve, refract over the trial lens and determine the power by adding the overcorrection to the known trial lens power. I recommend that you order warranted lenses because minimal changes often greatly improve final best corrected vision.

Even if the keratoconus patient has a significant amount of irregular corneal astigmatism, I never use a bitoric RGP design. Bitorics work well to correct regular astigmatism, but I believe they're of little benefit when correcting most corneal irregularities.

Follow-up should include a contact lens fitting examination every six months to monitor corneal integrity and measure corneal curvature. Keratoconus patients are quite concerned about the progression of the disease, so be sure to update them at follow-up visits. If a patient displays significant central corneal staining, decrease the base curve radius, thus steepening the lens fit. More times than not, the patient will enjoy a relatively stable lens wearing situation.

Most keratoconus patients want to avoid corneal transplantation at all costs, and many surgeons will refer a prospective penetrating keratoplasty patient for "one last try" with contact lenses before surgery.

SoftPerm (Wesley Jessen) lenses may provide relief for early-to-moderate keratoconus. A piggyback soft lens worn under an RGP may satisfy an uncomfortable patient. I use a custom soft contact lens from Alden Optical because it can be manufactured in a very steep base curve with a slightly smaller diameter (7.7mm/13.0mm).

Keratoconus patients require more chair time and lens designs that are more costly to manufacture. Consequently, fitting fees range from $150 to $250 per eye.

SEAMLESS COMANAGEMENT AFTER PK

I believe post-PK patients are the most challenging contact lens cases. Graft healing may take more than a year, and most surgeons prefer to leave sutures in place as long as possible. However, most patients want their best corrected visual acuity as soon as possible.

The contact lens practitioner and the corneal surgeon comanage the patient during healing and the subsequent contact lens fitting. Most surgeons allow a contact lens fitting before the one-year surgical anniversary, and some may permit the patient to wear a contact lens even though sutures remain. Be aware that wearing a contact lens may promote suture rupture, which should be handled by the surgeon.

Post-PK corneal topographies present a variety of fitting options. I sometimes trial fit a SoftPerm lens which may mask the resulting corneal astigmatism. If the lens fits well without the soft skirt buckling, the patient will report a very comfortable lens. Follow post-PK patients closely after surgery to monitor suture integrity and neovascularization, and to ensure that healing is progressing as it should. If you prescribe a SoftPerm lens, remember that the lens will likely cover both the host and the donor graft, and that the permeability of this lens is somewhat less than that of a conventional RGP.

When prescribing RGPs after PK, trial fitting is mandatory. Base the prescription on the central keratometric readings of the donor cornea. Most practitioners prescribe larger diameter, spherical lenses. Don't expect a perfect, upper lid attachment fit, and accept temporal or nasal displacement, as long as the lens doesn't dislodge easily. The post-PK patient is the only irregular cornea case for whom I'll consider prescribing a bitoric RGP lens because sometimes the astigmatism in the donor cornea is regular.

BE RESOURCEFUL AFTER TRAUMA OR INFECTION

Measurements of a cornea that has been traumatized through injury or infection can be very distorted, resulting in little practical fitting data. I measure the curvature of the unaffected fellow cornea and make one major assumption -- that the keratometric measurements were relatively equal before the injury. I order a warranted lens using the contralateral keratometric refraction, and I follow the patient more frequently than I would a conventional lens patient. Many of these patients will be wearing only one lens, so they will report lens sensation. Reassure them during adaptation, emphasizing the improvement in vision.

Many penetrating injury patients have also been left aphakic after the injury has been repaired. Since the mass of an aphakic lens is greater than that of a conventional lens, I initially fit these patients slightly steeper than the average corneal measurements. I recommend lenticular lens designs in 9.0mm diameters or larger in a fluorosilicone acrylate material.

FIG. 2: FLUORESCEIN IMAGE ILLUSTRATES APICAL TOUCH FIT.

FIG. 3: APICAL CLEARANCE WITH ADEQUATE EDGE LIFT.

FLUCTUATING VISION & DRY EYE AFTER RK

People seek refractive surgery so they won't have to wear spectacles or contact lenses. However, they may need to wear contact lenses part-time for sports or driving. If you explain this in a positive manner, patients will understand and appreciate the value of contact lenses for these specific tasks.

Post radial keratotomy patients are especially difficult to fit with contact lenses. Preoperative keratometry and manifest refraction are essential. This is one case where I may empirically fit or, at the very least, select the initial base curve from the preoperative information. I order these lenses warranted and may make slight fitting or prescription changes one to two weeks after initial dispensing. If preoperative measurements are not available, my only option is to trial fit based on postsurgical keratometry or postsurgical corneal maps. I select a base curve much steeper than the postsurgical central keratometric measurements because the midperipheral area is the target area of optimal fit. Many RGP laboratories will lend you post-RK fitting sets for special cases.

Lens centration is more of a challenge in the post-RK patient. Some special designs have secondary curves steeper than the base curve to solve centering problems. Order these lenses warranted and charge a higher fitting fee. Some post-RK patients report fluctuating vision which may be minimized by an RGP. Dryness associated with lens wear may be more severe after corneal surgery, so many RK patients become contact lens intolerant. If this is the case, consider a toric soft contact lens. Once you achieve a successful lens fit, follow the patient closely to monitor any corneal neovascularization.

A STRAIGHTFORWARD APPROACH AFTER PRK

Prescribing contact lenses after PRK is a somewhat more straightforward procedure. Many early PRK patients displayed spherical corneal topographies preoperatively. Some post-PRK patients are satisfied that their myopia has been reduced and are happy to wear their new prescription. Many of these patients may be corrected with a spherical soft contact lens. A more precise lens fit is required to achieve the maximal vault fitting relationship. Toric soft lenses are also an option to correct any residual post PRK refractive error. Again, more frequent follow-up is necessary to monitor corneal integrity and refractive changes.

Dr. McLaughlin is an assistant professor of clinical ophthalmology at The Ohio State University in Columbus. His practice is limited to contact lenses and sports vision analysis.